I think the fanny/lumbar pack is a nearly ideal format for a PSK. I do have a sling pack that I really want to love; it's a great bag with great organization, well made, etc. But it just doesn't carry that well for more technical hikes. It's ideal for a bag to have a vehicle to keep your stuff organized and that could be carried if need be...but it just isn't ideal for steep terrain or rough, rocky ground.

My newest "lumbar" pack is a Mystery Ranch Hot Shot pack. I got it used and pretty filthy but in great condition otherwise. I took it to a car wash and power washed it out, then let it dry in front of a fan. So far I'm very impressed! It has a real frame and a great belt and is supposed to be capable of carrying 70 lbs (although it's only about 1200 ci so no way you'd ever get that in the pack). After a couple of test hikes I may move the contents of my sling pack into the Mystery Ranch fire pack.